NM_000330.4(RS1):c.175T>G (p.Cys59Gly) is a missense variant encoding the substitution of cysteine with glycine at amino acid 59, which is a critical amino acid residue involved in disulfide bridge formation as defined by the ClinGen X-linked IRD VCEP (PMID: 26812435, PM1_Strong). Another missense variant in the same codon, NM_000330.4(RS1):c.175T>A (p.Cys59Ser), (PMIDs: 31174210, 10450864, 17987333, 9618178) has been classified as pathogenic for X-linked retinoschisis by the ClinGen X-linked IRD VCEP, while no benign missense variants have been identified in this codon. The present variant has a higher Grantham’s distance (159) than the comparison variant (112), and SpliceAI has been used to confirm that neither variant has a predicted impact on RS1 splicing (PM5). This variant is absent from hemizygous individuals in gnomAD v4.1.0 (PM2_Supporting). At least one proband harboring this variant exhibits a phenotype including appearance of schisis and reduced visual acuity before age 13 years, which together are specific for X-linked retinoschisis (PMID: 34645606, PMID: 33460243, PP4). This variant has been reported in at least 1 other proband meeting the PS4 requirement of a male diagnosed with X-linked retinoschisis, in addition to the apparently unrelated proband previously used for the PP4 code (PMIDs: 30652005, PS4_Supporting). The variant has been reported to segregate with X-linked retinoschisis through at least 3 meioses in two families with multiple affected brothers (PP1_Strong; PMID: 30652005). The computational predictor REVEL gives a score of 0.868, which is within the linGen X-linked IRD VCEP range between 0.931 to 0.773 and predicts a damaging effect on RS1 function. PP3 is not met since this code is ineligible in combination with PM1_Strong. The computational splicing predictor SpliceAI gives a delta score of 0.00 for all predictive splice changes, which is below the ClinGen X-linked IRD VCEP threshold of >0.2 and does not predict that the variant disrupts RS1 splicing. In summary, this variant is classified as pathogenic for X-linked retinoschisis based on the ClinGen X-linked Inherited Retinal Disease Expert Panel Specifications to the ACMG/AMP Variant Interpretation Guidelines for RS1 Version 1.0.0: PM2_Supporting, PM1_Strong, PP1_Strong, PP4, and PS4_Supporting.